Virtually all the flu in the United States this season is resistant to the leading antiviral drug Tamiflu, and scientists and health officials are trying to figure out why.

The problem is not yet a public health crisis because this has been a below-average flu season so far and the chief strain circulating is still susceptible to other drugs — but infectious disease specialists are worried nonetheless.

Last winter, about 11 percent of the throat swabs from patients with the most common type of flu that were sent to the Centers for Disease Control and Prevention for genetic typing showed a Tamiflu-resistant strain. This season, 99 percent do.

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“It’s quite shocking,” said Dr. Kent Sepkowitz, director of infection control at Memorial Sloan-Kettering Cancer Center in New York. “We’ve never lost an antimicrobial this fast. It blew me away.”

The single mutation that creates Tamiflu resistance appears to be spontaneous, and not a reaction to overuse of the drug. It may have occurred in Asia, and it was widespread in Europe last year.

[3] Is a potential drug-resistant swine flu pandemic on the horizon, or are health officials’ warnings just scare tactics?: http://www.prisonplanet.com/is-a-potential-drug-resistant-swine-flu-pandemic-on-the-horizon-or-are-health-officials-warnings-just-scare-tactics.html